Limestone is a natural crystalline mineral, which has as its major constituent calcium carbonate, characterized chemically as CaCO.sub.3. Limestone is used in many processes including chemical manufacturing, agricultural product manufacturing, and construction material manufacturing. In particular, limestone is used in power plant boilers for flue gas desulfurization or flue gas scrubbing, which is a process for removing sulfur-based compounds such as sulfur dioxide (SO.sub.2) from flue gas prior to discharge to the atmosphere. In the most basic exemplification of a flue gas desulfurization process, limestone is mixed with a liquid carrier such as water to create a slurry. The slurry is then transported to a scrubbing tower where it is injected into and intimately mixed with the flue gas stream which is discharged from the boiler. The intimate mixing of the limestone and flue gas at elevated temperatures fosters the desulfurization or scrubbing process.
Limestone is also used as a desulfurizing or scrubbing medium in fluidized bed combustion units. In these units, combustion materials such as coal and the like are fluidized and combusted in a combustion vessel by contact with upflowing high temperature gasses. In modern practice, limestone is mixed with the combustion materials prior to feeding of the mixture into the vessel, and the desulfurization process is effectuated in the vessel during combustion.
The scrubbing or desulfurization process is a chemical reaction. Therefore, the proper stoichiometric ratio of limestone containing calcium carbonate to flue gas will produce the most efficient reaction. In order for the desulfurization reaction to proceed efficiently and with minimal waste, the limestone must be processed such as by grinding to produce a defined particle size prior to use. In the fluidized bed combustion process, it is of utmost importance to control limestone particle size distribution to within stringent standards. If the particles are too large, the desulfurization process will not be efficient because there is insufficient limestone particle surface area to react with the flue gas. On the other hand, if the particles are small fines of limestone dust particles, generally smaller than 200 mesh, the limestone will be carried out of the vessel with the flue gas before it can react to remove the sulfur.
Hammer mills, vertical roll crushers, and other types of mills are typically used to grind limestone. However, the present grinding processes often produce a high quantity of fines, and further lack the ability to control the particle size distribution. Often, the quantity of fines which is produced in such grinding processes far exceeds that which is acceptable for certain uses such as desulfurization in the fluidized bed combustion process.
Roll grinders are not presently used in grinding processes for limestone, but, such grinders are used in grinding processes for milled products such as flour and the like. The grinders used in these processes typically comprise opposing rotating corrugated rolls which may rotate at different velocities relative to each other to produce a shearing force across the grinding surfaces of the rolls. This shear is imparted to the milled substance to produce a grinding effect. The grinding action occurs due to the hammer and anvil type crushing effect of the projections of one roll meshing with the corrugations of the other roll.
If used to grind limestone, the corrugated rolls would have a number of disadvantages. First, corrugated rolls would tend to "blind", that is the limestone would stick to the rolls and fill the corrugations. Consequently, the hammer and anvil type crushing effect of the rolls would be lost. Second, the projections on the corrugated rolls would tend to wear rapidly due to the abrasive action of the limestone against the rollers. This would reduce the effectiveness of the rolls and would tend to decrease the control over the final product particle size distribution. Thus, the nature of the limestone precludes the use of corrugated roll grinders.